High reliability, low-cost bonding wires are desirable for RF semiconductor packages. Bonding wires for such packages are typically made of gold, aluminum or copper. Some of these bonding wires function as tuning wires for an output match network connected to the RF power device. The tuning wires are subjected to significantly higher temperatures compared to the other bonding wires included in the package for input and output connections. For example, the temperature of the tuning wires often exceeds about 150° C. to 160° C., and even 200° C. in certain applications, due to RF Joule heating, i.e., ohmic heating and resistive heating whereby the tuning wires release heat as a result of electric current passing through the wires at RF frequencies.
Gold bonding wires can accommodate higher temperatures caused by RF Joule heating as compared to aluminum and copper bonding wires, but gold is significantly more expensive. Unprotected copper bonding wires readily oxidize in the presence of oxygen. Copper oxide growth is a function of temperature and time. The lifetime of an RF power device can, therefore, be predicted and meet minimum requirements. The critical (maximum) temperature for copper tuning wires depends on various conditions such as device electrical sensitivity, element, alloy, time and temperature, and is typically about 150° C., below which oxide growth is not problematic for most device useful lifetimes, e.g., 20 years. Copper bonding wires can be coated with an anti-oxidation layer such as palladium, but still oxidize at temperatures above 150° C. to 160° C. over long periods of time. Aluminum bonding wires are less sensitive to the temperature issues described above as compared to copper bonding wires, and have a self-passivating oxide layer that limits further oxidation. However, aluminum bonding wires have reduced electrical and thermal conductivity compared to copper and gold bonding wires. Fuse current is also significantly lower.
High bonding wire temperatures have been addressed in other ways for RF power packages. For example, the design time can be increased so that enough simulations can be run to yield reduced tuning wire temperatures. Integrated passive devices can be added to the package to realize the preferred matching, reducing the need for matching with tuning wires. The product can be de-rated to lower the current within the tuning wires. The number of bonding wires can be increased. Also, the bonding wire diameter can be increased. In each case, a better way of addressing the heating of certain bonding wires within an RF power package is desirable.